Novel Head-Disk Spacing Measurement Using Michelson Laser Interferometry through Glass Disk (Improvement in Accuracy by Considering Multiple Interferences over Stratified Slider Surface)

Abstract

A novel method using the Michelson laser interferometry was developed for measuring the head-disk spacing in a hard disk drive. This method utilizes the phase comparison of two kinds of interference fringe patterns formed respectively over the inner surface of a glass disk and the air bearing surface of a slider through the glass disk. To increase the accuracy in identifying fringe parameters, the fringe pattern was equivalently magnified by only one set of the fringes was selected to be formed on CCD, and the fringe interval was determined from the displacement of a high precision piezo actuator attached to the reference mirror. In this method, a phase shift generated by light reflection on the slider surface incurring multiple interference effects must be compensated. For this purpose, first the phase shift was evaluated by considering the multiple beam interferences over a stratified medium inclusive of the glass disk, the lubricating air film, the DLC protective layer, the silicon under layer and the substrate. Then, the shift was subtracted from the optical spacing to obtain the mechanical spacing. Finally, the repeatedly measured mechanical spacings were compared with the theoretically calculated spacings, showing a good accordance between them and a good reproducibility of less than a standard deviation of 1 nm.